Fusing device and method

ABSTRACT

A fusing device is described for fixing resinous powder images to a substrate such as paper comprising a rotating heating element in contact with a rotating backup member. Pressure means applies a meshing force between the heated member and backup member so that it is not necessary for the heated member in contact with the powder image to carry an offset preventing liquid on its surface. The fusing device is employed in reproduction apparatus, such as electrophotographic apparatus.

United States Patent Banks [451 May 30, 1972 [54] F USING DEVICE AND METHOD [21] Appi.No.: 888,666

[52] 11.8. CI ..263/6 E, 219/388 [51] Int. Cl. ...G03g 13/20, 603g 15/20 [58] Field otSeai-ch ..263/6 E, 3;219/388 [56] References Cited UNITED STATES PATENTS 3,256,002 6/1966 Hudson ..263/3 3,291,466 12/1966 Aser et a]. ..263/6 E 3,449,548 6/1969 Adamek et al. ..263/6 E X 3,452,181 6/ 1969 Stryjewski ....263/6 E X 3,498,596 3/1970 Moser ..263/ 6 E Primary Examiner-Charles J. Myhre Attorney-Hanifin and Jancin and Edward W. Brown [57] ABSTRACT A fusing device is described for fixing resinous powder images to a substrate such as paper comprising a rotating heating element in contact with a rotating backup member. Pressure means applies a meshing force between the heated member and backup member so that it is not necessary for the heated member in contact with the powder image to carry an offset preventing liquid on its surface. The fusing device is employed in reproduction apparatus, such as electrophotographic apparatus.

3Clalms,9Drawingflgures PATENTEDHAY 30 I972 v 3, 666,247 sum 1 or a FIG. 2B

INVENTOR WILLARD K. BANKS ATTORNEY FUSING DEVICE AND METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention.

This invention relates to a heat fusing method and devices for use in fixing images, especially resinous powder images to a support such'as paper and, more particularly, to heat fusing devices in which the heating element, such as a hot roller, is brought into physical contact with the image to be fixed.

2. Statement of Prior Art In order to fix or fuse images formed of powdered resins now commonly used in electrophotography, it is necessary to heat the powder and the paper to which it is to be fused above 210 C. This will vary depending on the softening range of the resin, but in any case, the temperature should be kept below 4l0 F because of the tendency of paper to discolor at such elevated temperatures.

' In lieu of the above method along with its disadvantages, faster and more efficient method of applying the heat for fusing the powderimage to paper is to have direct physical contact between a hot surface, such as a heated roller and the powder image so that the resin is heated to the necessary temperature for fusing.

However, this method has the disadvantage of ofiset. That is, as the resin powder is softened by the heat and becomes tackified, part of the image will stick to the surface of the hot surface such as the heated roller, so that when the next or second sheet comes into contact with the roller, the image partially removed from the first sheet will at least in part transfer to this sheet and, at the same time, part of the softened image of the second sheet will stick and transfer to the roller. Thus, all but the first sheet will be unacceptable copies.

This offset problem has been overcome in commercial machines by fabricating the outer surface of the heated roller of polytetrafluoroethylene or fluorinated ethylene/propylene and, most importantly, continuous coating the roller during fusing with an offset preventing liquid such as silicone oil. While this solution to the offset problem is sufiiciently feasible that it is used in commercial reproduction machines, it should be apparent that it adds complexity to the design of the fuser device in that it must include a reservoirfor providing a supply of offset preventing liquid and mechanism for continuously coating the roller with the liquid. Moreover, additional maintenance is required in that the reservoir of offset preventing liquid must be replenished periodically, which is added cost. Accordingly, thissolution to the offset problem is costly to'manufacture and maintain.

SUMMARY OF THE INVENTION It, therefore, is the principal object of this invention to pro vide a fusing device which makes physical contact with the toner image on the paper and which, without the use of an offset preventing liquid, offset is prevented.

Another object of the present invention is to provide such a fusing device which also is capable of operating at high speed and yet pennanently fuse the toner to the paper.

A further object of the invention is to provide such a fusing device which also operates as efficiently in transferring heat to the toner image as known fusing devices employing an offset li uid. I

These and other objects are achieved by a fusing device which comprises a rotatable member having a yieldable or compressable high temperature outer material with a dry surface for making contact with the image, means for heating said member, and a back-up member of a high temperature material, the support carrying the toner image passing between the two members. Pressure means is provided for applying sufficient force between the two members so that the compressable material of the heated member is compressed while the toner image is in contact but is returning to its uncompressed state at the point when theimage goes out of contact therewith. Preferably, the meshing force applied to the heated member either directly to or through the back-up member should be at least about 3 pounds per inch and the heated roll should be at a temperature sufficient to achieve a fuse quality of (the method for making this determination will be described subsequently) with conventional toner which has a fusing range of about 2l0250 F. More preferable, the heated and back-up members take the form of rolls and the outer material of the heated member is a high temperature elastomer, such as one comprising silicone rubber.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the fusing device of the present invention in conjunction with an electrophotographic apparatus.

FIGS. 2, 2A, 2B, and 2C show the the preferred roll embodiment of the fusing device with alternative heating means.

FIG. 3 shows a belt embodiment of the fusing device of the present invention.

FIG. 4 is a graph which shows the relationship of force and temperature to offset to the heated member with a fusing device of the present invention.

FIG. 5 is a graph which shows the relationship of temperature and force to fuse quality with a fusing device of the present invention.

FIG. 6 shows a circuit for controlling the temperature of the fusing device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS .Referring to FIG. 1, there is shown an electrophotographic apparatus embodying the fusing device of the present invention. The electrophotographic apparatus comprises a housing 1 having disposed therein a drum 2 mounted on a shaft 3 for rotation in the direction shown by the arrow and carrying a photoconductive member 4. Positioned around the drum are the various stations for performing the previously recited electrophotographic process steps comprising a cycle. At the first station, a corona charging unit 5 serves to lay down a uniform electrostatic charge. This unit preferably includes a wire filament array 6 which is maintained at a high potential as described, for example, in U.S. Pat. No. 2,588,699. At the next station, an original to be reproduced is placed face down on a transparent supporting member 7 which may be fabricated of glass, polymethylmethacrylate, or the like. The original is then flash illuminated by light sources 8 which desirably have back reflectors 9. Radiation from the light sources 8 is absorbed by the dark-colored portions, normally the print area of the original and reflected ofi the light colored portions, normally the background, of the original. The reflected radiation provides the exposure of a pattern corresponding to the pattern on the original. The next station comprises a developing unit 10 including an outer container or cover 11 with a trough 12 at its bottom containing a supply of developing material 13. This material comprises a mixture of commercially available electroscopic powder and carrier particles for the powder and is picked up from the bottom of the container 11 by a number of buckets 14 on an endless driven conveyor belt 15.

The carrier particles serve to deagglomerate the toner particles and to charge them by virtue of the relative position of toner and carrier material in the triboelectric series. Thus, friction between the toner powder and carrier particles during tumbling of the development mixture causes them to charge to opposite polarities so that the toner powder clings to the carrier particles. When the carrier particles with the toner powder clinging to them are cascaded across an electrostatic charge pattern, the electrostatic fields from the pattern pull the toner powder off the carrier particles and render the pattern visible. The carrier particles, along with some toner powder not used to develop the electrostatic pattern, fall back to the bottom or trough 12 of the container 1 l for reuse. Periodically, the toner powder is replenished by means not shown.

To transfer the now electroscopic powder pattern to a copy paper, a transfer station includes a supply of copy paper, preferably a stack of sheets 16, and a feed device 17 for top feeding single sheets of copy paper into contact with electroscopic powder pattern. Preferably, the transfer station includes a corona discharge unit 18 connected to a source of high potential so as to deposit a charge on the back of the copy sheet of the same polarity as the charge laid down by the corona discharge unit at the charging station. This electrostatic charge on the back of the copy sheet causes the attraction of the electroscopic powder pattern to the front of the copy sheet so as to temporarily affix the electroscopic copy pattern to the copy sheet. Because all of the electroscopic powder pattern is not transferred to the copy sheet, the last station which completes the electrophotographic cycle is a cleaning station including a cleaning brush 21 joumaled for rotation in a vacuum housing 22 connected to a vacuum source, not shown. If desired, an erase lamp (not shown) can be included in the electrophotographic cycle to dissipate any electrostatic charges remaining on the photoconductive member 4.

In accordance with the present invention, the electroscopic powder pattern which has been transferred to the copy sheet is fused to the sheet by passing the sheet through a fusing device 23 comprising at least one heated member 24 and a back-up member 25. Preferably, as shown in FIG. 2, the heated member 24 is in the shape of a roller and includes a heat conducting tubing 26, such as copper or aluminum, having infrared lamp 27 disposed therein and with deformable layer of an insulating, high temperature material such as one comprising silicone rubber. The wattage of the lamp will vary, depending upon the speed of the copy sheet through the fusing device. For example, at 32 inches per second, two infrared lamps mounted side by side are employed. The wattage of these lamps are 135 watts per inch. Herein, the copper tubing is 3 inches in diameter and one-eighth inch thick, but the thickness is not at all critical inasmuch as it is an excellent conductor of heat. However, the thickness of the deformable layer is somewhat critical in that it is an insulating material and heat flow or heat conductance decreases with increased thickness. In the present instance, it is about mils, but thicknesses as great as about 20 and as low as about 3 mils have been employed. The low limit on thickness is governed by strength and the ability of the material to deform under reasonable pressure. An advantage of a relative thick coating, i.e.- 10 mils, is that, if cascade development is employed in the electrophotographic process, carrier'beads which might be transferred to the copy paper will not damage the heated element 24. The remaining parameters are high temperature resistance and hardness. Preferably, the material should be able to continuously withstand at least up to about 550 F. without degradation, and should preferably have a shore A durometer of 30-35.

Turning now to the back-up element 25, which also preferably is in the shape of a roller, is herein pushed against the heated element 24 with a meshing force supplied by pressure means such as an air piston 29 driven by a pressure regulator (not shown). Preferably the back-up element is also formed of a high temperature elastomer 30 such as one comprising silicone rubber. However, the elastomer is of a higher durometer and thicker relative to the elastomer of the heated element. The durometer can range from 40-70, Shore A, and preferably is about 65. Herein, the elastomers are cast from a mold in the form of a roll and the shaft 31 is added to the center of the roll.

As an alternative embodiment of FIG. 2, the infrared lamp 27 can be replaced by a resistant heating element 32 as shown in FIG. 2A. The resistant heating element 32 can either be mounted in the center of the heating element 24 or disposed adjacent the periphery of the element 24 as shown in FIG. 2B. In the embodiment of FIG. 2B, the conductive tubing 26 can be eliminated and the casting of the elastomer roll containing the resistant heating element 32 near the surface can be mounted directly to a shaft 32. A further alternative of the roll embodiment is shown in FIG. 2C wherein both rolls 24, 24' contain heating elements. Herein, the heating elements are infrared lamps 27, 27', but, if desired, the heating elements and configurations of FIGS. 2A and 28 may be employed. The outer coating 28, 28 of the rolls is a high temperature elastomer. Again, one comprising silicone rubber is preferred. In addition, it will be noted that coating 28' on the back-up roll 24' is thicker than the coating on the other heated roll 24 so as to increase the contact zone during fusing. This coating 28 is also a higher durometer material, as was the other coatings on the back-up rolls of FIGS. 2-2B. In order to more clearly show the infrared lamp in the back-up roll, pressure means such as an air piston has not been shown.

Instead of the roll configuration of FIGS. 2-2C, the rotating members of the fusing device of the present invention can take the form of a pair of endless belts 34, 35, each mounted on a pair of shafts 36, 37, as shown in FIG. 3. Herein, a resistant heating element 38 is disposed in the belt 34 similar to the roll embodiment of FIG. 28. Both belts are of a high temperature elastomer, with the back-up belt 35 being of a higher durometer than the heated belt. Herein, the direction of travel of the belts 34, 35 is indicated by the arrows 39 so that the paper carrying the resinous powder or electroscopic toner travels from the entrance 40 of the fusing device to the exit 41, thereby increasing the time the heated belt is in contact with the paper. Thus, the power requirements are lower in the operation of this embodiment of the fusing device of the invention. Pressure means, which in the present instance is an air piston 42 driven by a pressure regulator (not shown), forces the two belts together at least at the exit 41 of the fusing device.

If desired, the back-up belt 35 may contain a heated element similar to the heated belt 34. Alternatively, the belt configuration of FIG. 3 may be modified to be an equivalent of the roll configurations of FIGS. 2, 2A and 2C.

While it is not completely understood why the fusing device of the present invention operates without an offset preventing liquid on surface of the heated member, one explanation is based on the fact that the elastic coating of the back-up member is of a higher durometer than the heated member. The elastic coating deforms during fusing of the resinous powder or electroscopic toner due to the applied meshing force. While the toner is still in a molten state and when the deformed elastomer is in the process of returning to its original shape, it separates from the toner thereby setting up a shear force which prevents the toner from offsetting to the heated element. That is, for relatively soft elastic rubber coating at high meshing force, a shear force develops. When a segment of the soft coating comes into initial contact with the harder back-up coating, the surface of the segment is near its normal radii, and surface friction dictates a one to one correspondence of surface areas. As contact progresses, the surface radius of the segment decreases. Poissons ratio suggests this radial compressive strain is accompanied by an orthogonal strain. A shear stress is a result of the frictional constraint. When paper is introduced, a similar stress is built up, but probably only on the back-up coating side.

Turning now to FIG. 4, there is shown a plot of hot roll and back-up roll offset boundaries on a domain of temperature versus meshing force. The fusing device of the present invention for obtaining the data for FIG. 4 comprises a heated roll of a 8-inch long, 1/16 inch thick copper tubing coated with G. E. RTV 106 silicone rubber having a durometer of 30, Shore A. Thus, the force in pounds are divided by 8 to obtain the force in pounds per inch, i.e.- at the valley of heated roll boundary, the force is 1.5 pounds per inch. The back-up roll was a solid cast G. E. RTV 11 having a 45 durometer, Shore A. During the test, the paper was fed'through the rolls at 20 inches per second for fusing conventional, commercially available toner images to 16 lb. bond paper. No offset preventing liquid was applied to either roll either before or during the test. The heated or hot roll offset boundary to the left is termed cold offset 43 and to the right is termed hot offset 44.

in the zone between cold and hot boundaries, there is no offset zone and fuse quality of at least 70 can be found based upon an abrasion test'to be described. At the cold ofiset boundary, often the heavy bulk of a character'will ofiset, leaving on the paper the background and the fringe edge of the character. The resinous powder or toner will typically warm up on the roll, and in one revolution, replant on the copy or the next sheet of paper. At the hot offset boundary, the individual toner particles of background and the fringe edge of the characters will often offset, leaving a well fused background free image. Hot offset will not clean easily; often solvent cleaning is necessary. Thus, because of the different characteristics between hot and cold offset, an examination of the fused image on the copy paper reveals when the meshing force and temperature are such that the fusing device of the present invention is operating at one of the boundaries and which boundary it is, thereby indicating the temperature adjustment or the pressure adjustment needed to place the operation in the no offset zone.

Another explanation as to why there is a no offset zone under predetermined conditions of temperature and pressure is that the coating on the heated element acts as a heat flow control member. Evidence for this explanation is found in the fact that offset occurred under all conditions of temperature and pressure when the heated element was an uncoated hot metal roll, even when an offset preventing liquid was used. With such a fusing device, the heat flow path is extremely short and the heat flow to the toner image is large. However, the fusing device of the present invention has a large no offset zone, especially with the paper feed through the fusing device with the toner image down. In this latter situation, the heat flow path to the toner image is long because it includes the thickness of the paper. Also, due to the temperature drops in passing through the coating of the heated member and the paper, the heat flow is substantially reduced as compared to the uncoated metal roll.

To evaluate the fuse quality of the fused toner image, the toner image is mechanically abraded with a piece of filter paper and then the filter paper is optically measured for the amount of toner transferred to the filter paper. That is, a clean piece of filter paper of known optical reflection is placed on a movable vacuum abrasion head, the vacuum holding the filter paper in position. The head is hinged to swing into and out of contact with a fixed vacuum platen on which a copy paper having a fused toner image is placed face up. A reflection device comprises an adjustable light source and solar cell detector with a calibrated meter ranging from -100. The light source is focused on the clean filter paper and its intensity is adjusted until reflected light to the solar cell causes a current of 100 microamperes for reading of 100 on the meter. Now, the abrasion head is swung down into contact with the copy paper and moved back and forth a predetermined number of times, such as 10, causing the filter to rub against the toner image.

After the predetermined number of times is completed, the abrasion head is swung out of contact with the copy paper. The reflection device is now turned on for reading of the optical reflection of the filter paper, the intensity of the light source being the same as before the abrasion test. A reading of 100 indicates a perfectly fused copy, while a reading of 30 indicates poorly fused copy. Satisfactory fuse quality is a reading of about 70 or above.

Shown in FIG. is a plot of temperature versus fuse quality with three different meshing forces. The fusing device of the present invention employed in obtaining this data is the configuration of FIG. 2 with the paper feeding through the rolls at 16 inches per second. The heated and back-up rolls were ll inches in length so that the meshing force of l 10 pounds, l50 pounds, l pounds is respectively 11 pounds per inch, l3.6 pounds per inch, and 16.8 pounds per inch. For this device, it will be noted that at pounds (ll pounds per inch) and 390 5., the fuse qualityuis below the satisfactory 70 level.

Wh e a number of own temperature controllers can be used with the fusing device of the present invention, FIG. 6 shows a block diagram of a Fenwel Series 56lXX controller. Two independent Wheatstone bridge circuits 45, one AC powered and one DC powered, utilize a temperature transducer, such as a thermistor 46, as a common leg. A DC power supply 47 provides power to the DC powered bridge which, in turn, provides an indicator signal to a temperature indicator or meter 48. An AC source 49 supplies power to the other bridge, which provides the control signal to a four stage amplifier 50. The amplified control signal operates the output relay 51 for controlling the intensity of the infrared lamp 52 and maintaining temperature at the level set in the controller.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In an electrophotographic apparatus, a fusing device for fixing a resinous powder image to a substrate,

a pair of first and second rotatable members, through which said substrate carrying the powder image passes and comes into contact with said first member, said first member having a high temperature resistant, elastic, compressible coating surrounding and in physical contact with a heat conducting support member, said second member being elastic and deformable by said first member when in pressure contact, the outer surfaces of both of said members being free of an offset preventing liquid,

heating means disposed within said support member for heating the outer surface of said elastic coating to an elevated temperature, the thickness of said elastic coating being such that heat from said heating source is readily conducted through said coating and is at least about 3 mils,

pressure means for applying a meshing force between said members at a pressure sufficiently high to cause said coating to be in a compressed state about said powder image and to deform said second member so as to create a contact zone, but sufficiently low to avoid detrimental effects to said substrate, and

means for rotating said members at a speed such that the powder image is in contact with said coating for a sufficient time to transfer sufficient heat to the powder image so as to raise the resinous powder to a temperature capable of fusing said powder image to said substrate above a predetermined minimum fuse quality without offsetting to said coating.

2. The fusing device of claim 1 wherein said pressure is up to and including about 16 pounds per inch.

3. The fusing device of claim 1 wherein the speed of said rotatable members in such that said substrate passes through at a speed of at least 16 inches per second. 

1. In an electrophotographic apparatus, a fusing device for fixing a resinous powder image to a substrate, a pair of first and second rotatable members, through which said substrate carrying the powder image passes and comes into contact with said first member, said first member having a high temperature resistant, elastic, compressible coating surrounding and in physical contact with a heat conducting support member, said second member beIng elastic and deformable by said first member when in pressure contact, the outer surfaces of both of said members being free of an offset preventing liquid, heating means disposed within said support member for heating the outer surface of said elastic coating to an elevated temperature, the thickness of said elastic coating being such that heat from said heating source is readily conducted through said coating and is at least about 3 mils, pressure means for applying a meshing force between said members at a pressure sufficiently high to cause said coating to be in a compressed state about said powder image and to deform said second member so as to create a contact zone, but sufficiently low to avoid detrimental effects to said substrate, and means for rotating said members at a speed such that the powder image is in contact with said coating for a sufficient time to transfer sufficient heat to the powder image so as to raise the resinous powder to a temperature capable of fusing said powder image to said substrate above a predetermined minimum fuse quality without offsetting to said coating.
 2. The fusing device of claim 1 wherein said pressure is up to and including about 16 pounds per inch.
 3. The fusing device of claim 1 wherein the speed of said rotatable members in such that said substrate passes through at a speed of at least 16 inches per second. 